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Unformatted text preview: the processes for the production of syngas described
above. A typical example is the transformation of γ-aluminium oxide to α-aluminium
oxide at high temperatures which is accompanied with a decrease of the surface area
from ~200 m2 for γ-aluminium oxide to ~2 m2 for α-aluminium oxide upon heating to
t~1100°C [72]. Such phase changes can result in the collapse of pores and the
encapsulation of the active phase within the new lattice or the coalescence of active
sites. Another example is the formation of NiAl2O4 from NiO on Al2O3 if steaming is
carried out in the absence of H 2 above 700°C. The spinel is not active in the steam
reforming process and more difficult to reduce. The most common and often most
important cause for the deactivation of the catalysts is coking. Coking can result in the
mechanical destruction of the catalyst, its deactivation by blockage of the active sites or
the blockage of the reactor. Even partial blockage of the reactor is deteriorating as it
results in uneven flow distributions causing overheating, which causes irreversible
damage to the reactor. 1.4.1. Coking The terms ‘coke’ and ‘coking’ are generic descriptions for a variety of carbonaceous
deposits. Coke is formed at relatively high temperatures and its chemical nature can
vary widely from graphitic carbon to condensed polymers and metal carbides [17, 18,
71, 72, 90, 91, 92]. Which type of coke is formed depends greatly on the type of the 29 1. Introduction ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff ff
reaction, e.g. gas phase or liquid phase process, and the presence of catalytic
elements, e.g. metallic surfaces or acidic centres. The properties of common forms of
coke are summarised in Table 5. Tab. 5: Various types of coke [72]
Type atomic carbon adsorbed on Temperature of C/H formation [°C ]
Ca Origin Gasification ratio >200 a easy surfaces
Cb polymeric amorphous film >300 »8 possible whisker whisker carbon usually with >300 »8 reasonably a metal particle on top
carbides m etal carbides easy
>300 reasonably
easy surface deposited on non catalytic >300 » 50 difficult or catalytic surfaces
gas phase formed in the gas phase >500 »8 difficult liquid phase deposited from heavy oils ~400 »1 difficult >1000 high difficult in the liquid phase
graphit...
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